<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>triangle</title>
</head>
<body>
    <script>
        let a = 'ssss'
        a.toString()
        a.splice()
        a.slice()
        a.charAt()
        a.padStart()
        a.padEnd()
        a.length
        a.match()
        a.fixed()
        a.trim()
        a.trimStart()
        a.trimEnd()
        a.toLOwerCase()
        a.toUpperCase()
        a.replace(/[A-Z]/g,letter=>_+letter.toLowerCase())
    </script>
<div class="box"></div>
<style>
    .box{
        width: 0;
        height: 0;
        border-left: 100px solid transparent;
        border-right: 100px solid transparent;
        border-top: 100px solid transparent;
        border-bottom: 100px solid black;
    }
</style>
</body>
</html>